Motor control system



April T N. H. WILLBY 2,078,649

MOTOR CONTROL SYSTEM Filed March 12, 1936 WITNESSES:

INVENTOR 7 M jag Norman H. MV/by Patented Apr. 27, 1937 UNITED STATESMOTOR CONTROL SYSTEM Norman H. Wlllby, Irwin, Pa assfgnor to West-'lflfllolse Electric & Manufacturing Company, East Pittsburgh, Pa., acorporation of Pennsyl- ApplicationMarch .12. 1936, Serial No. 68,454

13 Claims.

My invention relates, generally, to motor control systems and moreparticularly to systems for controlling the operation ofelectrically-propelled vehicles. I

An object of my invention, generally stated, is to provide a system forautomatically controlling both the acceleration and the deceleration ofan electrically-propelled vehicle which shall be simple and efiicient inoperation and which may be economically manufactured and installed.

A more specific object of my invention is to provide for decelerating anelectrically-propelled vehicle by means of dynamic braking.

Another object of my invention is to provide a quick-acting andsmoothly-operating dynamic galking system for an electrically-propelledve- A further object of my invention is to provide for controlling theoperation of a motor-driven accelerator during the acceleration and thedeceleration of an electrically propelled vehicle.

Other objects of the invention will be explained fully hereinafter orwill be apparent to those skilled in the art.

According to oneembodiment of the invention, both the acceleration andthe deceleration of an electrically-propelled vehicle are com trolledprimarily by an accelerator of the type described in Patent No.1,991,229, issued February 12, 1935 to L. G. Riley and assigned to theWestinghouse Electric & Manufacturing Company. The accelerator comprisesa circular copper bus inside of which are disposed a plurality ofcontact fingers which are progressively forced against the bus by a pairof revolving rollers driven by a pilot motor. One half of the contactfingers are connected to one section of a two-section resistor which isutilized to control the propelling motor current during bothacceleration and deceleration of the vehicle, the other half of thecontact fingers being connected to the other section of the resistor.The rate of acceleration and deceleration is controlled by governing thespeed of the pilot motor by means of limit relays which are responsiveto the main. motor current, the accelerator resistors being so connectedin the main motor circuits that they control the motor current duringboth acceleration and dynamic braking.

For a fuller understanding of the nature and objects of my invention,reference may be had to the following detailed description, taken inconjunction with the accompanying drawing, in which the single figure isa diagrammatic view of a control system embodying my invention.

Referring to the drawing, a pair of electric motors i and II may beutilized for propelling a vehicle (not shown T e motor III is providitand il ed with an armature winding l2 and a series field winding l3.Likewise, the motor H is provided with an armature winding I4 and aseries field winding l5. An electrically operated line switch is isprovided for connecting the motors l0 and H to a trolley it whichengages a power conductor ll that may be energized from any suitablesource of power, such as a generating station (not shown).

Both the acceleration and the deceleration of the motors l8 and H areprimarily controlled by a motor-(driven accelerator A which is of thesame type as the one described in the aforementioned Patent No.1,991,229. The accelerator A comprises a circular bus 98 inside of whichare disposed a plurality of contact fingers 23 to 2, inclusive, whichare progressively forced against the bus 68 by a pair of revolvingrollers it and 2d.

The rollers it and 2d are driven by a pilot motor 33 through shafts 44and 45 connected by bevel gears 46. The pilot motor 43 is provided withan armature winding ll and two field windings Q6 and 49, one for eachdirection of rotation. An electrical braking, which will be more fullydescribed hereinafter, is provided for quick stopping of the pilotmotor.

The accelerator A is provided with resistors i and for controlling thecurrent in the motors The resistor 58 is divided into a number ofsub-divisions which are connected to the odd-numbered contact fingersill to 3d, inclusive, and the resistor 52 is divided into subdivisionswhich are connected to the even-numbered contact fingers 22 to 40,inclusive. The accelerator resistor is divided into two sections and theone section grounded, in order to reduce the maximum voltage of themotors ill and ti above ground during dynamic braking.

As shown, a number of cam switches All, and A5 are located in theaccelerator and are actuated by the shaft 65. 2 i to $2 indicate thecontact fingers over which the rollers it and 28 travel while the camswitches are closed. The function of the cam switches in the controlsystem will be explained more fully hereinafter.

In addition to the accelerator and the cam switches, numerous otherswitches are provided for performing certain switching operations. Theseswitches include a switch Ml for connecting the motorsv i0 and l i tothe resistor 52 during acceleration, a switch M2 for connecting oneterminal of the motors directly to ground after a major portion of theresistors have been shunted from the motor circuit by the acceleratorrollers, switches BI and B2 for establishing dynamic braking connectionsfor the motors I II and H and a switch B3 tor connectinga battery Thereference numerals 52 in the motor circuit to cause a current tocirculate through the motors during coasting of the vehicle.

Drum controllers MC and BC are provided for controlling the motorconnections during acceleration and braking, respectively. Thesecontrollers are electrically interlocked to prevent improper operationoi' the equipment.

A current limit relay 54 functions to limit the motor current duringacceleration by regulating the operation or the accelerator A, thecontact members of the relay being disposed to control the operation ofthe pilot motor 43. A similar relay is provided for controlling thepilot motor during coasting and dynamic braking oi the vehicle. In orderthat the relay 5! will operate satisfactorily on both the relativelyhigh motor current produced during braking and the low circulatingcurrent produced during coasting, provision is made for recalibratingthe relay when the vehicle is coasting. This is accomplished byproviding the relay with an additional coil 56 which is energized by acurrent proportional to the motor current that circulates duringcoasting.

In order that the rate of acceleration may be controlled by the operatorwithin certain limits. the relay N is provided with a spring ,51 thetension of which is controlled by means or the controller MC. Likewise,the tension of a spring I on the relay 5! may be controlled by means ofthe controller BC to vary the rate of dynamic braking.

With a view to preventing a sudden rush of current upon the applicationof power or dynamic braking ai'ter coasting oi the vehicle, the smallcurrent, which is permitted to circulate through the motors Ill and IIduring coasting and which is proportional to the speed of the vehicle,is utilized to efi'ect spotting oi the accelerator A by changing theposition oi the rollers l0 and 20 to match the car speed. The currentwhich is permitted to circulate during coasting is oi such a low valuethat it has no. appreciable braking eifect. However, as statedhereinbefore, the relay 5 is recalibrated to be responsive to the lowcurrent and the relay is utilized to cause the pilot motor 43 to operatein the proper direction to advance or retract the rollers as the cargains or loses speed while coasting.

In order that the functioning of the foregoing apparatus may be moreclearly understood, the operation of the system will now be described inmore detail.

Assuming that a control switch ii has been closed. the motors l0 and IImay be connected to the power source by actuating the master control MCto the switching position, thereby closing the switchesLS and MI. Theenergizing circuit for the actuating coil may be traced from thepositive terminal oi a battery 02 through the switch ll, conductor 82,contact fingers N and ll bridged by a contact segment 0' on thecontroller BC, conductor 81. contact fingers I and"! bridged by acontact segment II on the controller MC, conductor 12, the actuatingcoil I3 of the switch LS, conductor ll, an interlock I! on the switchBI, and conductors l8, l1 and I. to the negative terminal ofthebatterytt j The energizing circuit (or the actuating coil of the switchMi, extends from a contactfinger l0, which engages the contact segmentll 01' the controller MC. through conductors II and 22, the actuatingcoil I2 01 the switch Mi and conductors Y spring 51 by means of of theswitch 15 I1 and II to the negative terminal of the battery '2.

The closing of the switches LS and MI connects the motors II and H tothe power conductor II in parallel-circuit relation and in series with amajor portion of the resistors BI. and 52 in the accelerator A. Thecircuit through the motor It may be traced from the power conducthroughthe trolley ll, conductor 81, contact members 85 of the switch U5,conductor I. the armature winding i2 0! the motor Iii, the series fieldwinding ll, conductors l1 and II, contact members 89 of the switch Ml,conductor II. a portion of the resistor 52, contact finger 22 on theaccelerator A, the bus 18, contact finger 2|, a portion of the resistor5|, conductor 22, a coil 93 on the limit relay N and conductor 04 to agrounded conductor 85. The circuit through the motor ll extends from theconductor 86 through conductor 96, the series field winding I! and thearmature winding ll of the motor II to the conductor 81 and thencethrough the circuit previously traced to the grounded conductor 95.

Since a major portion of the resistors II and 52 are connected in themotor circuits, the 'vehicle will be operated at a slow speed. If it isdesired to accelerate the vehicle, the master controller MC may beactuated to one of the accelerating positions, thereby energizing thepilot motor II to revolve the rollers it and 20 of the accelerator Awhich shunt the resistors SI and 52 from the motor circuit in astep-by-step manner. The energizing circuit for the pilot motor 0 may betraced from a contact finger 91, which engages the segment H of thecontroller MC.

through conductor 08, contact members it and ill on the limit relay N.conductor I02, a coil ill! of the relay N, conductor I, contact fingersI05 and Ill bridged by a segment III on the controller BC, conductorI00, the cam switch A2 on the accelerator A, conductors I" and III,. thefield winding 48, an armature winding 41 of the pilot motor 42, andconductors I1 and II to the negative terminal of the battery .2.

In. this manner the pilot motor 42 operates the accelerator A toaccelerate the vehicle under the control of the limit relay N, the rateof acceleration depending upon the tension applied to the the mastercontroller MC. When the accelerator is advanced to a position in whichsufilcient current fiows through the coil 93 of the limit relay 54 toovercome the force oi! the spring 51, the contact members II and ill orthe relayare separated. thereby disconnecting the pilot motor I frommitting an electrical braking to be established which quickly stops themotor 43. The braking circuit may be traced from one terminal oi thearmature winding 41 through the field winding 4!, conductors H2 and H3,a resistor H4, contact fingers H5 and I" bridged by a contact segmentIII on the controller MC and conductors I. and 11 to the other terminalor the armature winding 41. when the motor current decreases to a valuewhich permits the spring 81 to reclose the contact members It and III,the motor 48 is reenergized to advance the accelerator A, therebyaccelerating the vehicle.

In this manner, the rollers IO and 20 o! the accelerator A are advanceduntil those portions oi. the resistors II and I! which were originallyconnected in the motor circuit are shunted from the motor circuits, atwhich time the switch M2 is closed to connect one terminal oi the motorsII and II directly. to the grounded conductor 05 the battery 2 and per-I ductors I21 and I24,

through contact members III of the switch M2. The energizing circuit forthe actuating coil of the switch M2 may be traced from the previouslyenergized conductor II through the actuating coil H3, conductor I2I',the cam switch AI, conductor-s I22, 11 and 13 to the negative terminaloi the battery 52.

However, the pilot motor 43 will continue to advance the acceleratorrollers I and .until they depress contact fingers 4|. and 42.respectively, in order that themaximum amount of resistance H and 52will be available for controlling the motor current during dynamicbraking of the vehicle, it being understood that more resistance may berequired to control the motor current during dynamic braking than duringacceleration. since the motors may be operating at a higher speed duringdynamic braking than during normal acceleration of the vehicle.

If it is desired to permit the vehicle to coast, the motors III and IImay be disconnected from the power source by actuating the controller MCto the on position, thereby permitting the line switch LS and theswitches MI and M2 to open. However, the switches BI, B2 and B3 areclosed at this time in order to establish the dynamic brakingconnections for the motors III and II and to connect the battery 53 intothe motor circuit to cause a' small current to circulate through themotors during coasting which is utilized to match the position of theaccelerator A with the car speed, thereby insuring that the acceleratorwill be in the proper position to prevent an excessiveflow of currentthrough the motors III and I I in the event that power is reapplied tothe motors or dynamic braking is required.

The battery 53 is-also utilized to energize the field-winding of one ofthe motors to insure that the motor current will build up rapidly topro-. duce the dynamic braking eiiect quickly in the event that it isrequired to stop the vehicle. The

energizing circuit for. the actuating coil of the switch B3 may betraced from'a contact finger I23 on the controller MC through conductorsI24 and I25, an interlock I25 on the switch LS, conan interlock I25 on aswitch B2,-conductor I3I,the actuating coil I32 of the switch B3 andconductors I33, 11 and 13 to the negative terminal of the battery 52. Aparallel circuit for the coil I32 is also established from conductor I25through contact fingers I34 and I35, bridged by a segment I35 on thecontroller BC, to the conductor I3I.

Following the closing of the switch B3, the switches BI and B2 areclosed. The energizing circuit for the actuating coil of the switch BIextends from therpreviously energized conductor I25 throughthe'interlock I25 on the switch LS, conductor I31, the actuating coilI38, conductor I33, an interlock Ill onthe switch B3 and conductors I42,11 and I4 to the battery 52. The energizing circuit for the actuatingcoil'of the switch B2 extends from the previously energizedconductor.I28.through'the coil I43 to the con ductor I39 and thencethrough the circuit previously traced to the battery 52. A holdingcircuit for the actuating coils of the switches BI and B2 is establishedby the closing of an interlock I44 on the switch B2. The holding circuitmay be traced from the conductor I33 through the interlock I44 to theconductor I33 and thence through conductors I1 and 18 to the battery 52.

As stated hereinbefore, the closing of the switch B3 energizes the.field winding I5 of the motor II with a small current which is limitedby a resistor I45 and a portion of the accelerator resistor 5I. Thiscauses a current to circulate through the motors I 5 and II which isproportional to the vehicle speed, thereby providing a means forgoverning the position of the accelerator A while the vehicle iscoasting. The circuit through the field winding I5 may be traced fromthe positive terminal of the battery 53 through contact members I45 ofthe switch B3, the resistor I45, conductors I41 and 35, the fieldwinding I5, conductor I43, contact members I45 of the switch BI,conductor I5I, a coil I52 on the limit relay 55, conductor I53, aportion of the resistor SI, conductor 92, the coil 93 of the limit relay54 and conductors 34 and 95 I I5, conductor I48, contact members I49 ofthe switch BI, conductor I5I. the coil I52 of the limit relay 55,conductor I53, the resistor 5I, the bus I3, the resistor 52, conductorI54, contact members I of the switch B2, conductor I55 and a resistorI51 to the other terminal of the armature winding I 2. The circuitthrough the field winding I3 may be traced from one terminal of thearmature winding I4, through conductor I48, contact members I49 of theswitch BI, conductor I5I, the coil I52, conductor I53, the resistor 5I,the bus I8, the resistor 52, conductor I54, contact members I55,conductor I55, resistor I51, field winding I3 and conductor 81 to theother terminal of the armature winding I4.

As stated hereinbefore a small current is permitted to circulate throughthe motors III and II during coasting which is utilized to cause thelimit relay 55 to govern the position of the accelerator rollers I9 and20 to match the vehicle speed. In order that the limit relay 55 willfunc-.

tion properly on a small current, the relay is nected across theresistor I51, thereby being responsive to the circulating current. Theenergizing circuit for the coil 55 may be traced from one terminal ofthe resistor I51 through cunductor I58, contact fingers I59 and I 5I,bridged by a segment I52 on the controller BC; conductor I53, the coil55 and conductor I54 to the other terminal of the resistor I51.

It will be observed that the contact members of the relay 55 are soconnected in the circuit for the pilot motor 43 that the motor may beoperated in either direction to properly spot the accelerator A duringcoasting of the vehicle. If the circulating current is below apredetermined value, the contact members I55 and I55 of the relay 55 areengaged to operate the pilot motor in the reverse direction. The circuitthrough the pilot motor may be traced from a contact finger I51 on thecontroller MC, through conductors I55 and I59, contact members I55 andI55, conductor III, a coil I12 on the relay 55, conductors I13 and I14,the cam switch A3, conductor N2. the field winding 49, the armaturewinding 41 and conductors 11 and 13 to the battery 52.

(ill

If the vehicle gains in speed while coasting, thereby increasing thecirculating current, the relay I operates to close contact members I"and Ill, thereby operating the pilot motor in the forward direction. Theenergizing circuit for the pilot motor extends from the contact memberI'll through conductors lit and ill, the cam switch A2, conductors illand lil. field winding 48. the armature winding 41 and conductors I1 andII to the battery 82. In this manner, the relay "functions to spot theaccelerator A during coasting of the vehicle, thereby insuring that theproper amount of resistance will be available to control the motorcurrent in case it is desired to reconnect the motors to the powercircuit or to utilize dynamic braking to stop the vehicle. I

If it is desired to stop the vehicle by means of dynamic braking, thecontroller BC is actuated to one of the braking positions, in which casethe switch B3 is opened to disconnect the battery 53' from the motorcircuit. However, the switches BI and B2 remain closed to maintain thedynamic braking connections for the motors. The motor current duringdynamic braking is governed by the accelerator A in a manner similar tothat utilized during acceleration of the vehicle, the operation of thepilot motor it being under the control of the limit relay '5. It will benoted that the conductor I" is connected to the battery it throughcontact fingers ill and 04 on the braking controller BC and also theconductor ill is connected to the battery 02 through contact fingers illand N, thereby permittingthe system to function properly ,during dynamicbraking regardless of the position of the master controller MC.

A braking circuit for the pilot motor 43 similar to that utilized duringacceleration may be establishedto quickly stop the pilot motor 43duringdynamic braking. As previously stated, the

pilot motor is operated in the reverse direction while the contactmembers I" and l" of the relay 5 are closed thereby operating therollers l9 and in a direction to shunt the resistors Ii and 52 from themotor circuit during dynamic braking. However, when the main motorcurrent is of sufilcient value to operate the relay it to open thecontact members I" and I, a braking circuit is established for the pilotmotor 43 to stop it quickly without the aid of a mechanical brake. Thiscircuit may be traced from one terminal of the armature winding 41through the field winding 48, conductors Ill and ill, a resistor I'll,conductor Ill and either through contact fingers ll! and ii! on themaster controller MC and conductors l8 and 11 to the other terminal ofthe armature ll, or through conductor in and contact fingers I and II!bridged by segment I on the controller BC and conductors ill and II and11 to the other terminal of the armature 41. In this manner. the pilotmotor 43 will be quickly stopped by the operation of the limit relay Ii.

Ifitisdesiredtoeifectaneasyshutoffof power from the propelling motorsduring acceleration of the vehicle, the pilot motor I! may be reversedby means of the limit relay 4 to cause a portion of the resistors ii and2 to be inserted in the motor circuit before the opening of the lineswitch L8 to disconnect the motors from the power source. This may beaccomplished bractuating the controller MC toward, but not entirely to,the switching position, thereby releasing thetensionontbespringll ontherelayll. In

this manner the contact member 99 of the relay N is permitted to engagea contact member I, thereby reversing the direction of the pilot motor43 which will cause a portion of the resistors ii and I! to be insertedin the motor circuit. The reversing circuit for the pilot motor I may betraced from the contact member I" on the limit relay N to conductors I13and ill, the cam switch Al, the field winding 49, the armature winding41 and conductors l1 and It to the negative terminal oi the battery 82.As stated hereinbeiore when the master controller is operated to the oilposition, the switches LS. Ml and M! are opened to completely disconnectthe motors ill and ii from the power source.

From the foregoing description, it is apparent that I have provided aquick acting and smoothly operating dynamic braking system for anelectrically propelled vehicle which is simple and emcient in operation.I have also provided a control system for an electrically propelledvehicle in which a motor driven accelerator is utilized to control thepropelling motor current both during acceleration and deceleration ofthe vehicle, and I have devised a simple and emcient method ofcontrolling the operation of the motor driven accelerator.

Since many modifications may be made in the apparatus and arrangement ofparts without departing from the spirit of my invention, I do not wishto be limited other than by the scope of the appended claims.

I claim as my invention:

1. In a motor control system, in combination, a motor for propelling avehicle, a source of power for the motor, switching means for connectingthe motor to the power source, switching means for establishing dynamicbraking connections for the motor, a separate source of power forcausing a current to circulate through the motor during coasting of thevehicle, and means responsive to the circulating current for regulatingsaid current.

2. In a motor control system, in combination, a plurality of motors forpropelling a vehicle, a source of power for the motors, switching meansfor connecting the motors to the power source, switching means forestablishing dynamic braking connections for the motors, a separatesource of power for causing a current to circulate through the motorsduring coasting of the vehicle, and

means responsive to the circulating current for regulating said current.

3. In a motor control system, in combination, a plurality of motors forpropelling a vehicle, a source of power for the motors, switching meansfor connecting the motors to the power source, resistance-varying meansfor controlling the motor current during acceleration of the vehicle.switching means for establishing dynamic braking connections for themotors, a separate source of power for causing a current to circulatethrough the motors during coasting of the vehicle, and means responsiveto said circulating current for controlling the operation of theresistance-varying means.

4. In a motor control system, in combination. a plurality of motors forpropelling a vehicle, a source of power for the motors, switching meansfor connecting the motors to the power source, resistance-varying meansfor controlling the motor current during acceleration of the vehicle,switching means for establishing dynamic braking connections for themotors, a separate source of power'for causing a current to circulatethrough the motors din-ing coasting of the vdiicle,

said. 7.5

resistance-varying means being utilized to control the motor currentduring dynamic braking, and means responsive to said circulating currentfor controlling the operation of the resistance-varying means duringcoasting of the vehicle.

5. In a motor control system, in combination, a plurality of motors forpropelling a vehicle, a source of power for the motors, switching meansfor connecting the motors to the power source, resistance-varying meansfor controlling the motor current during acceleration of the vehicle,switching means for establishing dynamic braking connections for themotors, a separate source of power for causing a current to circulatethrough the motors during coasting of the vehicle, relay meansresponsive to said circulating current for controlling the operation ofthe resistance-varying means during coasting of the vehicle, and meansfor recalibrating said relay means while the vehicle is in operation.

6. In a motor control system, in combination, a plurality of motors forpropelling a vehicle, a source of power for the motors, switching meansfor connecting the motors to the power source, resistance-varying meansfor controlling the motor current during acceleration of the vehicle,switching means for establishing dynamic braking connections for themotors, a separate source .of power for causing a current to circulatethrough the motors during coasting of the vehicle, saidresistance-varying means being utilized to control the motor currentduring dynamic braking, relay means for controlling the operation of theresistance-varying means, and means for recalibrating the relay meanswhen the vehicle is coasting.

7. In a motor control system, in combination, a plurality of motorsforpropelling avehicle, a source of power for the motors, switchingmeans for connecting the motors to the power source, a master controllerfor controlling the operation of said switching means,resistance-varying means for controlling the motor current duringacceleration of the vehicle, a relay responsive to the motor current forgoverning the operation of the resistance-varying means, the operationof said relay being governed by said master controller. switching meansfor establishing dynamic braking connections for the motors, a brakingcontroller for controlling the operation of said dynamic-brakingswitching means, said resistance-varying means being utilized to controlthe motor current during dynamic braking, and a relay responsive to thedynamic-braking current for governing the operation of saidresistance-varying means during dynamic braking, the operation of saidrelay being governed by the braking controller.

8. In a motor control system, in combination, a plurality of motors forpropelling a vehicle, a source of power for the motors, switching meansfor connecting the motors to the power source, a master controller forcontrolling the operation of said switching means, resistance-varyingmeans for controlling the motor current during acceleration of thevehicle, a relay responsive to the motor current for governing theoperation of the resistance-varying means, the operation of said relaybeing governed by said master controller, switching means forestablishing dynamic-braking connections for the motors, a brakingcontroller for controlling the operation of said dynamic-brakingswitching means, said resistance-varying means being utilized to controlthe motor current during dynamic braking, a relay responsive to thedynamic-braking current for governing the operation of saidresistance-varying means during dynamic braking, the operation of saidrelay being governed by the braking controller, and means forrecalibrating the last mentioned relay to be responsive to a low currentwhen the vehicle is coasting.

9. In a motor control system, in combination, a motor for propelling avehicle, switching means for connecting the motor to a source of power,variable-resistance means for controlling the motor current, meansfor'operating the variableresistance means, a current-limit relay forcon trolling the operation of the last-named means, a controller forgoverning the operation of the relay and said switching means, and meansactuated by the limit relay for reversing thevariableresistance-operating means prior to the opening of the switchingmeans to disconnect the motor from the power source.

10. In a motor control system, in combination, a motor for propelling avehicle, switching means for connecting the motor to a source of power,variable-resistance means for controlling the motor current, a pilotmotor for operating the variable-resistance means, a limit relayresponsive to the motor current for controlling the operation of thepilot motor, a master controller for governing the operation of thelimit relay and said switching means, and means associated with themaster controller and the limit relay for reversing the pilot motorbefore the switching means is opened to disconnect the 'motor from thepower source.

11. In a motor control system, in combination, a motor for propelling avehicle, a variableresistance means for controlling the motor current, apilot motor for operating the variableresistance means, a limit relayresponsive to the motor current for controlling the operation of thepilot motor, and means actuated by the limit relay for reversing thepilot motor and for controlling the establishing of an electricalbraking circuit for the pilot motor.

12. In a motor control system, in combination a motor for propelling avehicle, variable-resist ance means for controlling the motor current, apilot motor for operating the variable-resistance means, a limit relayresponsive to the motor current for controlling the operation of thepilot motor, and contact members on the relay for reversing the pilotmotor and for controlling the establishing of an electrical brakingcircuit to stop the pilot motor.

13. In a motor control system, in combination, a motor for propelling avehicle, variable-resistance means for controlling the motor current, apilot motor for operating the variable-resistance means, a limit relayresponsive to the motor current for controlling the operation of thepilot motor. said pilot motor having a field winding for'each directionof rotation, and contact members on the relay for controlling theenergization oi said field windings to reverse the pilot motor, saidcontact members being disposed to control the establishing of anelectrical braking circuit for the pilot motor.

NORMAN H. WILLBY.

